Influenza remains an important cause of morbidity and mortality worldwide. In the United States alone, seasonal (inter pandemic) influenza is responsible for more than 200,000 hospitalizations and more than 23,000 deaths per year. The primary strategy to attenuate the impact of influenza epidemics is vaccination. Currently, the Advisory Committee on Immunization Practices (ACIP) recommends annual influenza vaccination for all persons aged 6 months or older. However, some of these recommendations have been questioned, and the paucity of vaccine effectiveness estimates based on laboratory confirmed influenza disease further fuels this controversy. Furthermore, because influenza viruses and vaccines change annually, and because recommendations for these vaccines now cover most of the US population, annual assessments of vaccine effectiveness are needed to assess whether influenza vaccination is having its intended effects. Vanderbilt investigators have conducted prospective population- based surveillance for laboratory confirmed influenza for both the NIH and the CDC. We propose to conduct prospective surveillance in Davidson County, TN, with the following specific aims: 1) To conduct active population-based surveillance in adults hospitalized for laboratory confirmed influenza, to determine age-specific rates of influenza-associated hospitalizations; and, 2) To determine influenza vaccine effectiveness annually, using a case-control study design, among influenza positive adults hospitalized for influenza and influenza negative hospitalized controls. Vanderbilt investigators are experienced in conducting active prospective population-based hospital surveillance of laboratory confirmed influenza infections and in assessments of the effectiveness of influenza vaccines.